The present embodiments relate to wireless communications systems and, more particularly, to transmitters with multiple transmit antennas used in such systems.
Wireless communications have become very prevalent in business, personal, and other applications, and as a result the technology for such communications continues to advance in various areas. One such advancement includes the use of spread spectrum communications, including that of code division multiple access (“CDMA”) and wideband code division multiple access (“WCDMA”) cellular communications. In such communications, a user station (e.g., a hand held cellular phone) communicates with a base station, where typically the base station corresponds to a “cell.”
Due to various factors including the fact that CDMA communications are along a wireless medium, an originally transmitted communication from a base station to a user station may arrive at the user station at multiple and different times. Each different arriving signal that is based on the same original communication is said to have a diversity with respect to other arriving signals originating from the same transmitted communication. Further, various diversity types may occur in CDMA communications, and the CDMA art strives to ultimately receive and process the originally transmitted data by exploiting the effects on each signal that are caused by the one or more diversities affecting the signal.
One type of CDMA diversity occurs because a transmitted signal from the base station is reflected by objects such as the ground, mountains, buildings, and other things that it contacts. As a result, a same single transmitted communication may arrive at the receiver at numerous different times, and assuming that each such arrival is sufficiently separated in time, then each different arriving signal is said to travel along a different channel and arrive as a different “path.” These multiple signals are referred to in the art as multiple paths or multipaths. Several multipaths may eventually arrive at the user station and the channel traveled by each may cause each path to have a different phase, amplitude, and signal-to-noise ratio (“SNR”). Accordingly, for one communication between one base station and one user station, each multipath is a replica of the same user information, and each path is said to have time diversity relative to other mulitpath(s) due to the difference in arrival time which causes different (uncorrelated) fading/noise characteristics for each multipath. Although multipaths carry the same user information to the receiver, they may be separately recognized by the receiver based on the timing of arrival of each multipath. More particularly, CDMA communications are modulated using a spreading code which consists of a series of binary pulses, and this code runs at a higher rate than the symbol data rate and determines the actual transmission bandwidth. In the current industry, each piece of CDMA signal transmitted according to this code is said to be a “chip,” where each chip corresponds to an element in the CDMA code. Thus, the chip frequency defines the rate of the CDMA code. Given the use of transmission of the CDMA signal using chips, then multipaths separated in time by more than one of these chips are distinguishable at the receiver because of the low autocorrelations of CDMA codes as known in the art.
In contrast to multipath diversity which is a natural phenomenon, other types of diversity are sometimes designed into CDMA systems in an effort to improve SNR, thereby improving other data accuracy measures (e.g., bit error rate (“BER”), frame error rate (“FER”), and symbol error rate (“SER”)). An example of such a designed diversity scheme is antenna diversity and is introduced here since it has particular application to the preferred embodiments discussed later. Antenna diversity, or sometimes referred to as antenna array diversity, describes a wireless system using more than one antenna by a same station. Antenna diversity often proves useful because fading is independent across different antennas. Further, the notion of a station using multiple antennas is more typically associated with a base station using multiple antennas to receive signals transmitted from a single-antenna mobile station, although more recently systems have been proposed for a base station using multiple antennas to transmit signals transmitted to a single-antenna mobile station. Each of these alternatives is further explored below.
Certain antenna array diversity techniques suggest the use of more than one antenna at the receiver, and this approach is termed receive antenna diversity. For example, in prior art analog systems, often a base station receiver was equipped with two antennas, each for receiving a signal from a single-antenna mobile station. Thus, when the single-antenna mobile station transmits to the base station, each receiver antenna provides at least one corresponding received signal for processing. By implementing multiple receive antennas, the performance of an ideal receiver is enhanced because each corresponding received signal may be separately processed and combined for greater data accuracy.
More recently there have been proposals to use more than one antenna at the transmitter, and this approach is termed transmit antenna diversity. For example, in the field of mobile communications, a base station transmitter is equipped with two antennas for transmitting to a single-antenna mobile station. The use of multiple antennas at the base station for transmitting has been viewed as favorable over using multiple antennas at the mobile station because typically the mobile station is in the form of a hand-held or comparable device, and it is desirable for such a device to have lower power and processing requirements as compared to those at the base station. Thus, the reduced resources of the mobile station are less supportive of multiple antennas, whereas the relatively high-powered base station more readily lends itself to antenna diversity. In any event, transmit antenna diversity also provides a form of diversity from which SNR may be improved over single antenna communications by separately processing and combining the diverse signals for greater data accuracy at the receiver. Also in connection with transmit antenna diversity and to further contrast it with multipath diversity described above, note that the multiple transmit antennas at a single station are typically within several meters (e.g., three to four meters) of one another, and this spatial relationship is also sometimes referred to as providing spatial diversity. Given the spatial diversity distance, the same signal transmitted by each antenna will arrive at a destination (assuming no other diversity) at respective times that relate to the distance between the transmitting antennas. However, the difference between these times is considerably smaller than the width of a chip and, thus, the arriving signals are not separately distinguishable in the same manner as are multipaths described above.
Given the development of transmit antenna diversity schemes, two types of signal communication techniques have evolved to improve data recognition at the receiver given the transmit antenna diversity, namely, closed loop transmit diversity and open loop transmit diversity. Both closed loop transmit diversity and open loop transmit diversity have been implemented in various forms, but in all events the difference between the two schemes may be stated with respect to feedback. Specifically, a closed loop transmit diversity system includes a feedback communication channel while an open loop transmit diversity system does not. Both of these systems as well as the distinction between them are further detailed below.
FIG. 1 illustrates a prior art closed loop transmit antenna diversity system 10 including a transmitter 12 and a receiver 14. By way of example, assume that transmitter 12 is a base station while receiver 14 is a mobile station. Also, for the sake of simplifying the discussion, each of these components is discussed separately below. Lastly, note that the closed loop technique implemented by system 10 is sometimes referred to in the art as a transmit adaptive array (“TxAA”), while other closed loop techniques also should be ascertainable by one skilled in the art.
Transmitter 12 receives information bits Bi at an input to a channel encoder 13. Channel encoder 13 encodes the information bits Bi in an effort to improve raw bit error rate. Various encoding techniques may be used by channel encoder 13 and as applied to bits Bi, with examples including the use of convolutional code, block code, turbo code, or a combination of any of these codes. The encoded output of channel encoder 13 is coupled to the input of an interleaver 15. Interleaver 15 operates with respect to a block of encoded bits and shuffles the ordering of those bits so that the combination of this operation with the encoding by channel encoder 13 exploits the time diversity of the information. For example, one shuffling technique that may be performed by interleaver 15 is to receive bits in a matrix fashion such that bits are received into a matrix in a row-by-row fashion, and then those bits are output from the matrix to a symbol mapper 16 in a column-by-column fashion. Symbol mapper 16 then converts its input bits to symbols, designated generally as Si. The converted symbols Si may take various forms, such as quadrature phase shift keying (“QPSK”) symbols, binary phase shift keying (“BPSK”) symbols, or quadrature amplitude modulation (“QAM”) sybmols. In any event, symbols Si may represent various information such as user data symbols, as well as pilot symbols and control symbols such as transmit power control (“PC”) symbols and rate information (“RI”) symbols. Symbols Si are coupled to a modulator 18. Modulator 18 modulates each data symbol by combining it with, or multiplying it times, a CDMA spreading sequence which can be a pseudonoise (“PN”) digital signal or PN code or other spreading codes (i.e., it utilizes spread spectrum technology). In any event, the spreading sequence facilitates simultaneous transmission of information over a common channel by assigning each of the transmitted signals a unique code during transmision. Further, this unique code makes the simultaneously transmitted signals over the same bandwidth disinguishble at receiver 14 (or other receivers). Modulator 18 has two outputs, a first output 181 connected to a multiplier 201 and a second output 182 connected to a multiplier 202. Each of multipliers 201 and 202 multiplies its input times a weight value, W1 and W2, respectively, and provides an output to a respective transmit antenna A121 and A122. By way of example, assume that transmit antennas A121 and A122 are approximately three to four meters apart from one another.
Receiver 14 includes a receive antenna A141 for receiving communications from both of transmit antennas A121 and A122. Recall that such communications may pass by various multipaths, and due to the spatial relationship of transmit antennas A121 and A122, each multipath may include a communication from both transmit antenna A121 and transmit antenna A122. In the illustration of FIG. 1, a total of j multipaths are shown. Further, each multipath will have a fading channel parameter associated with it, that is, some value that reflects the channel effects on the signal carried by the channel. By way of reference, the character α is used in this document to identify this fading parameter; moreover, in FIG. 1, the convention αij is used, where i=1 identifies a path transmitted by the antenna A121, i=2 identifies a path transmitted by the antenna A122, and j identifies the multipath. Within receiver 14, signals received by antenna A141 are connected to a despreader 22. Despreader 22 operates according to known principles, such as by multiplying the CDMA signal times the CDMA code for receiver 14, thereby producing a despread symbol stream at its output and at the symbol rate. The despread signals output by despreader 22 are coupled to an open loop diversity decoder 23, and also to a channel estimator 24. Channel estimator 24 determines estimated channel impulse responses based on the incoming despread data. Further, channel estimator 24 provides two outputs. A first output 241 from channel estimator 24 outputs the estimated channel impulse responses to open loop diversity decoder 23. In response to receiving the estimates, open loop diversity decoder 23 applies the estimates to the despread data received from despreader 22; further in this regard and although not separately shown, the application of the estimate to the data may be by way of various methods, such as maximal ratio combining (MRC) and using a rake receiver. A second output 242 from channel estimator 24 communicates the estimates, or values derived from those estimates, back to transmitter 12 via a feedback channel. These feedback values are the weights W1 and W2 described above with respect to multipliers 201 and 202 of transmitter 12.
Returning to open loop diversity decoder 23 of receiver 14, once it applies the estimates to the despread data, its result is output to a deinterleaver 25 which operates to perform an inverse of the function of interleaver 15, and the output of deinterleaver 25 is connected to a channel decoder 26. Channel decoder 26 may include a Viterbi decoder, a turbo decoder, a block decoder (e.g., Reed-Solomon decoding), or still other appropriate decoding schemes as known in the art. In any event, channel decoder 26 further decodes the data received at its input, typically operating with respect to certain error correcting codes, and it outputs a resulting stream of decoded symbols. Indeed, note that the probability of error for data input to channel decoder 26 is far greater than that after processing and output by channel decoder 26. For example, under current standards, the probability of error in the output of channel decoder 26 may be between 10−3 and 10−6. Finally, the decoded symbol stream output by channel decoder 26 may be received and processed by additional circuitry in receiver 14, although such circuitry is not shown in FIG. 1 so as to simplify the present illustration and discussion.
Having detailed system 10, attention is now returned to its identification as a closed loop system. Specifically, system 10 is named a closed loop system because, in addition to the data communication channels from transmitter 12 to receiver 14, system 10 includes the feedback communication channel for communicating weights W1 and W2 from receiver 14 to transmitter 12; thus, the data communication and feedback communication channels create a circular and, hence, “closed” loop system. Note further that weights W1 and W2 may reflect various channel affecting aspects. For example, receiver 14 may ascertain a level of fading in signals it receives from transmitter 12, such as may be caused by local interference and other causes such as the Doppler rate of receiver 14 (as a mobile station), and in any event where the fading may be characterized by Rayleigh fading. As a result, receiver 14 feeds back weights W1 and W2 and these weights are used by multipliers 201 and 202, thereby applying weight W1 to various symbols to provide a transmitted signal along transmitter antenna A121 and applying weight W2 to various symbols to provide a transmitted signal along transmitter antenna A122. Thus, for a first symbol S1 to be transmitted by station 12, it is transmitted as part of a product W1S1 along transmitter antenna A121 and also as part of a product W2S2 along transmitter antenna A122. By way of illustration, therefore, these weighted products are also shown in FIG. 1 along their respective antennas.
Turning now to a prior art open loop transmit diversity system, it may described generally and in comparison to the closed loop system 10 of FIG. 1, where the primary distinction is that the prior art open loop transmit diversity system does not require feedback. Thus, to depict an open loop system the illustration of FIG. 1 may be modified by removing the feedback channel, weights W1 and W2 and multipliers 201 and 202, with the remaining blocks thereby generally illustrating an open loop transmit diversity system. Given that the open loop transmit diversity system does not include feedback, it instead employs an alternative technique to adjust data differently for each of its transmit antennas. Therefore, the open loop system receiver then attempts to properly evaluate the data in view of the known transmitter adjustment. Thus, the processing and algorithms implemented within the receiver decoder of an open loop system will differ from those in a closed loop system.
To further depict open loop transmit diversity, FIG. 2 illustrates, by way of example, a prior art open loop transmitter 30 that is referred to as providing space time block coded transmit antenna diversity (“STTD”), and further in this regard transmitter 30 includes an STTD encoder 32. STTD encoder 32 has an input 34, which by way of example is shown to receive a first symbol S1 at a time T followed by a second symbol S2 at a time 2T. For the sake of the present example, assume that symbols S1 and S2 are QPSK symbols. STTD encoder 32 has two outputs 361 and 362, each connected to a respective antenna A321 and A322.
The operation of transmitter 30 is now explored, and recall in general from above that open loop system transmitters adjust data differently at each transmit antenna without the assistance of feedback. In the case of transmitter 30, STTD encoder 32 first buffers a number of symbols equal to the number of transmit antennas. In the example of FIG. 2 which has two transmit antennas A321 and A322, STTD encoder 32 therefore buffers two symbols (e.g., S1 and S2). Next, STTD encoder 32 directly transmits the buffered symbols along antenna A321 and, thus, in FIG. 2 symbol S1 is transmitted at a time T′ and symbol S2 is transmitted at a time 2T′. During the same time, however, and for transmission along antenna A322, the complex conjugates of the symbols are formed and reversed in order. For the example of FIG. 2, therefore, these two operations create, in the reversed order, a sequence of S*2 and S*1. Moreover, when transmitted along antenna A322, the negative value of the first of these two symbols is transmitted while the positive value of the second symbol is transmitted. Accordingly, in FIG. 2 and with respect to antenna A322, a symbol −S*2 is transmitted at a time T′ and a symbol S*1 is transmitted at a time 2T′. From the symbols transmitted by STTD encoder 32, a compatible receiver is therefore able to resolve the symbols in a manner that often yields favorable data error rates even given relatively large Doppler rates. Finally, note also by way of an alternative example that if symbols S1 and S2 were BPSK symbols, then such symbols would include only real components (i.e., they do not include a complex component). In this case, along antenna A321 system 30 would transmit symbol S1 at time T′ and symbol S2 at time 2T′, while along antenna A322 system 30 would transmit symbol S2 at time T′ and symbol −S1 at time 2T′.
Having detailed both closed loop and open loop transmit antenna diversity systems, additional observations are now made regarding the benefits and drawbacks of each. In general, under the ideal situation, a closed loop system outperforms an open loop system for a given transmitted signal power. However, due to non-ideal occurrences in the feedback information, a closed loop system may be inferior to an open loop system in some situations. For example, as Doppler fading increases, by the time the feedback information is received by the transmitter, the weights included or derived from the feedback information may be relatively outdated and therefore less effective when applied to future transmissions by the transmitter. Conversely, because the open loop system does not implement feedback from the receiver to the transmitter, then such a system may provide greater performance in a high Doppler environment. In the prior art, the drawbacks of both the closed loop and open loop systems have been addressed in one manner by further increasing the number of antennas in either the closed loop or open loop system. While this approach may improve error rates as compared to fewer antennas for the same system, there are diminishing returns in data error rates to be considered versus the complexities of adding more antennas to a system. Moreover, for each antenna added to a closed loop diversity system, there is a corresponding increase in the amount of bandwidth required to accommodate the additional feedback information required for the added transmit antenna.
In view of the above, there arises a need to improve upon the drawbacks of prior art closed loop systems and prior art open loop Systems, and such a need is addressed by the preferred embodiments described below.